An electrical or power substation includes electricity transmission and distribution systems where high and low voltages are stepped down or up, respectively using transformers. Electric power may flow through several substations between generating plant and consumer or load, and may be transformed in voltage in several steps.
Known substations can include a plurality of power and distribution transformers, cabling, switching, reactive power and grounding equipment. The equipment should be protected against power system anomalies can using Intelligent Electronic Devices (IEDs). Currently different types of known IEDs are used in a substation for functions related to protection, control, monitoring and metering. The IEDs are microprocessor-based equipment associated with power distribution system equipment, such as circuit breakers, generators, transformers, power lines, power cables, reactors, motors, capacitor banks, or other suitable components as desired. The IEDs receive primary power distribution system information such as voltage and current from sensors to perform various protection and monitoring functions. Manual and Automatic control command functions like closing and opening of switching equipment (circuit breakers and disconnectors), or raise/lower voltage levels in order to maintain the desired voltage levels. Common types of IEDs include protective relaying devices, load tap changer controllers, circuit breaker controllers, recloser controllers, voltage regulators, DFRs (Digital Fault Recorders), DSRs (Digital Surge Recorders) and programmable logic controllers (PLCs) etc.
Thus a single unit can perform several protections, metering, monitoring, and control functions concurrently.
The IEC61850 standard is an open communication standard from International Electrotechnical Commission (IEC) that advocates interoperability amongst Intelligent Electronic Devices (IEDs) from various manufacturers using common engineering models, data formats and communication protocol. Recent IEDs are therefore designed to support the IEC61850 standard for substation automation, which provides interoperability and advanced communications capabilities like GOOSE (Generic Object Oriented Substation Event) and MMS (Manufacturing Message System) communication profiles.
In the current power distribution systems, the causes for power distribution system reliability conditions, such as the fault conditions, can be analyzed through waveform records stored in Intelligent Electronic Devices (IEDs). Ideally, waveform records should be stored by IEDs during pre-fault and post fault conditions. This means that IEDs should have a data storage mechanism, and means to continuously monitor field data and trigger the waveform storage once the fault is detected. The recorded data represents the condition before and during occurrence of the fault for analyzing the conditions that led to the fault. Hence, the waveform recording is an elemental component of a substation automation system.
The current power distribution systems, however, are not able to perform waveform recording in a desired manner due to various limitations. The IEDs that record the waveform data could either be a protection/control IED that performs protection and control related actions as its primary function and recording as a secondary function, or a dedicated IED performing only the waveform recording functionality. When the IED is used for waveform recording, as a secondary function, in known implementations the IED can have less waveform storage space compared to a dedicated waveform recorder IED. Due to memory limitations, duration of the pre-fault and post-fault records in the waveform may get restricted. Besides that, the protection devices most often operate at lower sampling rates leading to a low resolution waveform recording. This condition can cause limitations in root cause analysis and create dependencies on the dedicated waveform recorder IED which is comparatively a costly solution.
In some known power distribution systems, the waveform recording function present in protection IEDs can sample from 4 to 128 samples/cycle, the sampling rate can be limited because of input filters providing filtered quantities for main protection and control functions. Dedicated DFRs (with or without process bus input) and standalone DFRs can be used for higher sampling rates such as, at 64 to 356 samples/cycle. Thus, the recording device may have limitations on its sampling capabilities. Further, the instrument transformers in a substation are hardwired with the DFRs, which are time synchronized with a time server at different synchronization resolutions. Thus, the fault waveforms recorded by these substation automation devices in a distributed manner may result in timing inaccuracies.
There are different conditions that occur within the power distribution system which demand high sampling rates of digitized data for a shorter duration. Moreover, conditions can arise where lower sampling rates of digitized voltage and/or current data are used for a longer duration. Under these conditions, it can be desirable to have flexibility in a recording device so that it can support both high sampling rates of digitized voltage and/or current data and have a capacity to store these data for a long duration.
With multiple IEDs in the system, the available resource utilization can be improved to support waveform recording at desired sampling rates and for desired time durations instead of using a dedicated DFR which may have a time synchronization problem when hardwired with the instrument transformer and time server for sampled measured value and time data acquisition, respectively. This configuration can result in deviations in the timing information of disturbance data recorder in distributed devices. With a Merging Unit present in the process bus, the usage of dedicated DFRs can be expensive and also reduce the availability of the overall system as it contributes to an increase in the number of automation elements resulting in reduced Mean Time Between Failure (MTBF).
A flexible method for recording waveforms should include accurate time synchronization and efficient use of the resources in the power distribution system.